This project is evaluating the associations between differences in monoamine oxidase activity and human or animal behavior. In continuing studies of individuals in the upper and lower 10 percent of MAO activity from a sample of 375 normals, rapid eye movement (REM) sleep and cortical evoked response differences have been shown to be associated with platelet MAO differences. Studies of factors capable of affecting platelet MAO have shown that smaller, lighter platelets have less MAO activity than larger, heavier platelets; however, most patients and normals with low MAO activity were not found to have a larger proportion of small, light platelets than controls. Marijuana smokers have lower platelet MAO activity than controls, but short-term THC administration or its addition to the enzyme in vitro did not reveal any significant inhibitory effect. These findings, in general, are consistent with our previous observations in patients and normals suggesting that reduced platelet MAO activity is associated with differences in personality features, psychobiological characteristics and a tendency towards increased vulnerability to psychopathology.